Cytokines

Abstract

Cytokines designate a broad category of factors that are produced by immune cells and/or act on immune cells. They are small proteins or peptides, mostly glycosylated, that regulate cellular growth, differentiation, and/or activity in a para- or autocrine manner. Those peptides, which are involved in growth and differentiation of target cells, are also named growth factors. Others regulate immune or inflammatory reactions and, in order to differentiate them from other factors, can be called mediators. However, there are examples for cytokines able to do both, and also factors other than cytokines can be assorted into these groups. The majority of cytokines are not exclusively synthesized by a single specific cell type. Moreover, certain cytokines are produced by both immune and nonimmune cells. On the other hand, a given cytokine can act on several cell types. And again, both immune and nonimmune cells may be affected by the same cytokine. Generally, cytokines are most important in regulatory processes of the immune system and can be classified into five subgroups: interleukins, tumor necrosis factors, interferons, chemokines, and colony-stimulating factors. This, however, is not an unambiguous and broadly accepted definition or classification of cytokines, but it is rather used to give this chapter a structure.